Apparatuses and methods for utilizing non-ideal light sources

ABSTRACT

A method includes obtaining a measurement of a property of a light source, scanning light from the light source onto a surface, such that the light interacts with the surface, detecting light from the surface to create a picture element, and correcting the picture element with the measurement of the property. An apparatus includes a scanned beam display, the scanned beam display is configured to receive a signal and to scan the signal for viewing by a user. The signal is to contain picture element information. The picture element information includes information for a plurality of colors, wherein information for at least one color is corrected to substantially remove a perturbation to the picture element information, such that an image containing the picture element information will be substantially unchanged by the perturbation.

RELATED APPLICATIONS

This application is a continuation-in-part of co-pending, commonlyassigned U.S. patent application Ser. No. 10/687,414, filed on Oct. 14,2003, entitled “Image Capture Device with Projected Display,” which is acontinuation of U.S. Pat. No. 6,661,393, which is a continuation of U.S.Pat. No. 6,445,362.

U.S. patent application Ser. No. 10/687,414, filed on Oct. 14, 2003,entitled “Image Capture Device with Projected Display,” and U.S. Pat.No. 6,661,393 are hereby incorporated by reference into the presentapplication.

BACKGROUND OF THE INVENTION

1. Field of Invention

The invention relates generally to non-ideal light sources, and morespecifically to methods and apparatuses for utilizing non-ideal lightsources in scanned beam devices, such as displays and imaging devices.

2. Art Background

Light sources are used in devices used for image capture and in devicesused to display an image to a user. Two parameters of a light source,used for such devices, are amplitude stability of the optical output asa function of time and the general stability of the dominant wavelengthof the spectral distribution of optical energy. Fluctuations of eitherone or both of these parameters can cause a number of undesirableeffects in either the image captured or the image displayed.

Several variations (non-idealities) of light source amplitude (opticalintensity) are shown in FIG. 1, where amplitude 102 is plotted as afunction of time 104. The preferred optical output is stable over timeas indicated at 106. Many real light sources exhibit one or more of thenon-idealities illustrated in FIG. 1 and FIG. 2.

Some examples of light sources used in the devices described above are alight emitting diode (LED), an edge emitting light emitting diode(EELED), a laser diode (LD), a diode pumped solid state (DPSS) laser,etc. One non-ideality these devices can exhibit is shot noise as shownat 114. Another non-ideality is a temporally periodic amplitudefluctuation 112. The amplitude can also decay with time as shown at 108or increase with time as shown at 110. The amplitude non-idealities arenot meant to be plotted on a common time scale 104 in FIG. 1, but aremerely over plotted on the same time scale 104 for ease of discussion.

An optical output, spectral power density, (SPD) 202 of a light sourceis plotted a function of wavelength 204 in FIG. 2. Light sources such asInGaN-based blue and green edge emitting light emitting diodes (EELEDs),LDs, as well as other light sources, exhibit a drive level dependentoutput spectra. Such a drive level dependent output spectra causes afirst spectrum with a dominant wavelength 206 to shift to a secondspectrum with a dominant wavelength 208. This may cause various problemswhen such a light source is used in devices used to capture or displayimages.

For example, FIG. 3 shows a shifting color gamut, generally at 300, dueto the effects described in conjunction with FIG. 2. With reference toFIG. 3, red, green, and blue (RGB) light source outputs 308, 310, and312 are plotted in a Commission Internationale de I'Eclairage (CIE)color space with y_(c) corresponding to 302 and x_(c) corresponding to304, and a region interior to curve 306 indicates the envelope of colorsperceivable to the human eye. The triangle formed by connecting 308,310, and 312 has a white point WP at 314. These RGB values cancorrespond to a particular light source drive level and 310 (FIG. 3) cancorrespond with dominant wavelength 206 (FIG. 2). At another drivelevel, the spectral output of the green source shifts to 210 (FIG. 2)causing a shift in the green dominant wavelength G plotted at 310 to aperturbed spectrum G′ plotted at 316. In this example, the peakwavelength of the green source not only shifted, but the spectral purityof the source also shifted, as shown by the shorter, broader spectrum210. Similarly, the blue light source can experience a shift in dominantwavelength from B to B′ with drive level, resulting in a shift of CIEchromaticity from 312 to 318. Ignoring the relative values of green andblue spectra, the change in spectral output corresponds, for example, toa shift from curve 206 to 208 (FIG. 2). A new color gamut is formed bythe triangle represented by 308, 316, and 318 (in this example the redlight source is assumed to be unaffected by drive level changes). Thewhite point WP′ within the new color gamut corresponds to point 320,which represents a shift from the white point WP at 314. Such drivelevel dependent color gamut fluctuations are often undesirable.

It is generally desirable to use inexpensive light sources in order toreduce the cost of a display or an image capture device. However,inexpensive light sources may tend to exhibit the above describednon-idealities to an unacceptable level. For example, an inexpensiveDPSS laser can exhibit amplitude fluctuations of 30 percent. Theamplitude noise on some DPSS lasers occurs in the 1-100 kilohertz band,which can coincide with the periodicity or other features of some imagedata, to make the image artifacts even more pronounced during eitherimage capture or image display.

Polarization can also fluctuate and, owing to polarization sensitivitiesin some systems, can produce similar amplitude variations. Typically,such amplitude variations result from polarization dependent differencesin system gain. Additionally, DPSS and similar sources may exhibit modecoupling, where the beating of a plurality of modes can produceamplitude noise.

While the tolerance of the eye to amplitude variations is imagedependent and spatial frequency dependent, the human eye generally candetect two (2) to three (3) percent amplitude variations in smallregions. Thus, amplitude variations in such light sources may produceperceivable image artifacts.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may best be understood by referring to the followingdescription and accompanying drawings that are used to illustrateembodiments of the invention. The invention is illustrated by way ofexample in the embodiments and is not limited in the figures of theaccompanying drawings, in which like references indicate similarelements.

FIG. 1 depicts temporal variations of a light source output.

FIG. 2 depicts a shift in a dominant wavelength of a light sourceoutput.

FIG. 3 shows a shifting color gamut due to the effects shown in FIG. 2.

FIG. 4A illustrates amplitude correction according to one embodiment ofthe invention.

FIG. 4B depicts a scanner according to one embodiment of the invention.

FIG. 4C illustrates amplitude correction according to another embodimentof the invention.

FIG. 5 illustrates amplitude correction utilizing feedback to a lightsource drive according to one embodiment of the invention.

FIG. 6 depicts self-calibration of light source amplitude according toone embodiment of the invention.

FIG. 7A illustrates methods of controlling a beam of light according toembodiments of the invention.

FIG. 7B illustrates a variable optical window according to oneembodiment of the invention.

FIG. 8 illustrates removal of noise components from an image accordingto one embodiment of the invention.

FIG. 9 shows correction of light source non-idealities in a device usedto display an image according to one embodiment of the invention.

FIG. 10 is a block diagram of a first system for generating an outputclock to retrieve data from a memory matrix while compensating fornonlinear scan speed of the resonant mirror, according to one embodimentof the invention.

FIG. 11 is a block diagram of an alternative embodiment of the apparatusof the block diagram of FIG. 10.

FIG. 12 shows a shift in a dominant wavelength of an InGaN-based greenedge emitting light emitting diode (EELED) at two different drivelevels.

FIG. 13 shows a shift of in a color gamut for a device utilizing red,green, and blue EELEDs.

FIG. 14 is a flow chart of a technique used to perform color correctionaccording to one embodiment of the invention.

FIG. 15 illustrates correction of the white point of a color gamut, as afunction of luminance level, according to one embodiment of theinvention.

FIG. 16 is a block diagram for a technique used to generate the lookuptable of FIG. 15, according to one embodiment of the invention.

FIG. 17 illustrates correction of the white point of a color gamut at afixed luminance level, according to one embodiment of the invention.

FIG. 18 illustrates a technique to generate Commission Internationale deI'Eclairage (CIE) chromaticity color coordinates from the systemprimaries, according to one embodiment of the invention.

FIG. 19 illustrates a technique to adjust a corrected white point whilemaintaining constant luminance, according to one embodiment of theinvention.

FIG. 20 illustrates a method to correct changes in a white balance of adisplay, according to one embodiment of the invention.

FIG. 21 shows individual RGB gamma corrections according to the methodof FIG. 20, for one embodiment of the invention.

FIG. 22 illustrates correcting effects of spectral power distribution(SPD) variations, according to one embodiment of the invention.

FIG. 23 illustrates various devices used to obtain information on thespectral power distribution (SPD) of a light source, according toembodiments of the invention.

DETAILED DESCRIPTION

In the following detailed description of embodiments of the invention,reference is made to the accompanying drawings in which like referencesindicate similar elements, and in which is shown by way of illustration,specific embodiments in which the invention may be practiced. Theseembodiments are described in sufficient detail to enable those of skillin the art to practice the invention. In other instances, well-knowncircuits, structures, and techniques have not been shown in detail inorder not to obscure the understanding of this description. Thefollowing detailed description is, therefore, not to be taken in alimiting sense, and the scope of the invention is defined only by theappended claims.

Apparatuses and methods are disclosed that facilitate utilization ofnon-ideal light sources in devices and systems that capture and/ordisplay image(s). In various embodiments, compensation for light sourcenon-idealities such as amplitude noise, dominant wavelengthfluctuations, light source color gamut correction, etc. are disclosed.In one or more embodiments, the non-idealities are known a priori. Inone or more embodiments, the non-idealities are not known a priori. Inone or more embodiments, combinations of non-idealities are presentwhere certain non-idealities are known a priori while othernon-idealities are not known a priori.

FIG. 4A illustrates amplitude correction according to one embodiment ofthe invention. A device capable of capturing an image is shown generallyat 400. With reference to FIG. 4A, a light source 402, creates a firstbeam of light 404. A scanner 406, which includes a mirror, deflects thefirst beam of light 404 to produce a scanned beam 403, the scanned beam403 passes through a transmissive window defined between surfaces 408 aand 408 b to illuminate a spot 411 on a surface 410. The scanner 406will be described more fully below in conjunction with FIG. 4B. Whilethe scanned beam 403 illuminates the spot 411 the scanned beam 403 isreflected, absorbed, scattered, refracted or otherwise affected by theproperties of the surface 410 at spot 411 to produce scattered lightenergy 412 from the spot 411. A portion of the scattered light energy412 is received by one or more detectors 414 and is typically convertedinto an electrical signal that may be analog or digital. In variousembodiments, the detector 414 is a photodetector. In other embodiments,the detector 414 senses a parameter related to the scattered lightenergy such as current flow, etc. The output of the detector 414 isreceived at block 420. Optionally, a position of the scanner,corresponding to the location of spot 411, is communicated at 424 toblock 420. A measure of the intensity of the light source, obtainedbefore the light source is scattered by the surface 410, is obtained, inone embodiment, with a device 422. In one embodiment, the device 422 isa partially transmissive mirror. The device 422 diverts a portion of thefirst beam of light 404 to a detector 426. The output of the detector426 is received at block 420. In one embodiment, the scattered lightenergy 412 is normalized by the intensity of the light source obtainedbefore the light beam is scattered by the surface 410 to produce acorrected scattered-light-energy value corresponding to the light energyscattered from the spot 411. The process is repeated for each spot onthe surface 410 corresponding to the collection of spots that composethe image of the surface 410. Correction of the scattered light energy412 accounts for various non-idealities of the light source 402, such asa time varying light source intensity. In various embodiments, thecorrection can be applied to a picture element (pixel), a group ofpixels, a line of an image, a frame of an image, etc.

According to various embodiments, the image capture device 400 can beused in a variety of applications, such as but not limited to, a digitalcamera, a bar code reader, multidimensional symbol reader, documentscanner, scanning endoscope, confocal microscope, confocal microprobe,or other image capture or acquisition device. To allow the device togather light efficiently, the device 400 can include gathering optics(not shown) that collect and transmit light from the surface 410 to thedevice 400. The gathering optics are configured to have a depth offield, focal length, field of view (FOV), and other opticalcharacteristics appropriate for the particular application. For example,in one embodiment, where the device 400 is a two-dimensional symbologyreader, the gathering optics may be optimized for red or infrared lightand the focal length may be on the order of 10-50 centimeters. Forreading symbols at a greater distance, the focusing optics may have alonger focusing distance or may have a variable focus. The optics may bepositioned at various locations along the optical path to allow smaller,cheaper components to be used.

While in many instances, non-imaging gathering optics are used, otherapplications may make use of a photodetector comprised of a number ofpixel elements such as a charge-coupled device (CCD) or a complimentarymetal oxide semiconductor (CMOS) focal plane imager. In such cases, thegathering optics are configured to provide an image of the surface 410,the scanning beam being used as primary or auxiliary illumination orexcitation.

FIG. 4B depicts a scanner according to one embodiment of the invention,illustrated generally at 430. With reference to FIG. 4B, a biaxialscanner 436 is a single mirror device that oscillates about twoorthogonal axes. Design, fabrication and operation of such scanners aredescribed for example in U.S. Pat. No. 5,629,790 to Neukermans et al.entitled MICROMACHINED TORSIONAL SCANNER, in Asada, et al, SiliconMicromachined Two-Dimensional Galvano Optical Scanner, IEEE Transactionson Magnetics, Vol. 30, No. 6, 4647-4649, November 1994, and in Kiang etal, Micromachined Microscanners for Optical Scanning, SPIE proceedingson Miniaturized Systems with Micro-Optics and Micromachines II, Vol.3008, February 1997, pp. 82-90 each of which is incorporated herein byreference. The biaxial scanner 436 includes integral sensors 434 thatprovide electrical feedback of the mirror position to terminals 438, asis described in U.S. Pat. No. 5,648,618 to Neukermans et al. entitledMICROMACHINED HINGE HAVING AN INTEGRAL TORSIONAL SENSOR, which isincorporated herein by reference. Torsional movement of amicroelectromechanical (MEMs) scanner can produce electrical outputscorresponding to the position of the scanner/mirror 406 (FIG. 4A). Lightfrom the light source 432 strikes the biaxial scanner 436 and is scannedhorizontally and vertically to approximate a raster pattern across asurface to be imaged, such as the surface 410 (FIG. 4A). Electricaloutputs, corresponding to the position of the scanner 436, arecommunicated optionally at 424 (as described above in conjunction withFIG. 4A) to provide the position of the spot 411 within the collectionof spots used to compose the image of the surface 410.

Alternatively, the position of the scanner/mirror may be obtained bymounting piezoelectric sensors to the scanner, as described in U.S. Pat.No. 5,694,237 to Melville, entitled POSITION DETECTION OF MECHANICALRESONANT SCANNER MIRROR, which is incorporated herein by reference. Inother alternatives, a position of the optical beam can be determined byan optical detector that monitors the scanned beam as the beam passesthrough a field of view (FOV). Alternatively, a line within an image orsuccessive lines within an image can be analyzed mathematically toobtain information corresponding to picture elements (pixels) withoutneeding the scanner mirror position sensors described above.

In one embodiment, a biaxial MEMs scanner 436 scans one or more beams oflight across a two-dimensional FOV or a selected region within a FOV tocapture a frame of an image. A typical frame rate is 60 Hz. In oneembodiment, both scanner axes can be operated resonantly. In anotherembodiment, one axis is operated resonantly, while the other axis isoperated non-resonantly in an approximation of a saw tooth pattern;thereby creating a progressive scan pattern. In one embodiment, aprogressively scanned bidirectional scanner, scanning a single beam isscanned at a frequency of 19 kilohertz in the horizontal and is scannedvertically in a sawtooth pattern at 60 Hz; thereby, approximating SuperVideo Graphics Array (SVGA) graphics resolution. In one embodiment ofsuch a system, the horizontal scanner is driven electrostatically andthe vertical scan motion is driven magnetically. Alternatively, both thehorizontal scan motion and the vertical scan motion may be drivenmagnetically or capacitively. In various embodiments, both axes of thescanner may be driven sinusoidally or resonantly.

With reference back to FIG. 4A, light source 402 can represent a singlelight source or a plurality of light sources. Various devices can beused for light source 402, such as but not limited to, a light emittingdiode (LED), an edge emitting light emitting diode (EELED), adiode-pumped solid state (DPSS) laser, a laser diode, a laser, a thermalsource, an arc source, a fluorescent source, a gas discharge source, orother types of light sources. In one embodiment, the light source 402 isa red laser diode, which has a wavelength of approximately 635 to 670nanometers (nm). In another embodiment, the light source 402 includes ared laser diode, a green DPSS laser, and a blue DPSS laser atapproximately 635 nm, 532 nm, and 473 nm respectively. Many laser diodescan be modulated directly; however, DPSS lasers often require externalmodulation, such as with an acoustic-optic modulator (AOM) for example.In the case where an external modulator is used, it is considered to bepart of the light source 402.

The light source 402 may include, beam combining optics (not shown) tocombine some or all of the light beams into a single beam. The lightsource 402 may also include beam-shaping optics (not shown) such as oneor more collimating lenses and/or apertures. Additionally, while thewavelengths described in the previous embodiments have been in theoptically visible range, other wavelengths are within the scope ofembodiments of the invention.

As described above, multiple light sources can be used for the lightsource 402. The first beam of light 404 can include a plurality of beamsof light converging on a single scanner 406 or onto separate scanners406.

Different types of detectors can be used for the detector 414. In oneembodiment, the detector 414 is a positive-intrinsic-negative (PIN)photodiode. In the case of multi-colored imaging, the detector 414 caninclude splitting and filtering functions to separate the scatteredlight into component parts before detection. Other detectors can beused, such as but not limited to, an avalanche photodiode, (APD) or aphotomultiplier tube (PMT). In some embodiments, the detector 414collects light through filters (not shown) to reduce ambient light.According to various embodiments, the detector(s) can be arranged tostare at the entire field of view (FOV), a portion of the FOV, collectlight retrocollectively, or collect light confocally.

Alternatively, non-optical detection may be used for monitoringvariations in light source output. For example, some variations in lightoutput may accompany or arise from changes in light source electricalcharacteristics. An increase in light emitter or circuit resistance, forexample, may result in decreased current dissipation; which in turn mayresult in decreased light output. In such an exemplary case, lightoutput variations may be correlated with electrical changes. Thus, anelectrical detector may be effectively used to monitor light output. Asillustrated by this example, the term “detector” may be used toencompass a range of technologies which detect a characteristic that iscorrelated to light output.

In various embodiments, the device 400 can be implemented to produce amonochrome or color image(s). The term “grayscale” will be understood torefer to embodiments of each within the scope of the teaching herein.Color images can be created utilizing a set of system primaries, such aswith nominally red, blue, and green (RGB) light sources or with variousother combinations of light sources. The system primaries need not beconstrained to the RGB triad.

As described above, a measure of the intensity of the light source 402is obtained before the scanned beam of light 403 interacts with thesurface 410. Accordingly, the measure of the intensity can be picked-offat any location along the optical path before the scanned beam of light403 interacts with the surface 410. In one or more embodiments, thedetector can be a detector similar to the description of detector 414.

FIG. 4C illustrates amplitude correction according to another embodimentof the invention, illustrated generally at 450. With reference to FIG.4C, a light source 452, creates a beam of light 454. A scanner 456,which includes a mirror, deflects the beam of light 454 to produce ascanned beam 453, the scanned beam 453 passes through an aperturedefined by surfaces 458 a and 458 b to illuminate a spot 461 on asurface 460. The scanner 456 can be a bidirectional scanner such as thescanner described above in conjunction with FIG. 4B or the scanner 456can include more than one scanner to scan a beam in a raster patternacross the surface 460. As described above in conjunction with FIG. 4A,the scanned beam is scattered from a spot 461 on the surface 460 toproduce scattered light energy 462.

The scattered light energy 462 is received by one or more detectors 464and is converted into a digital signal at analog to digital (A/D)converter 466. Appropriate filtering can be applied to the signal eitherdigitally after conversion by the A/D converter 466 or the analog signalcan be filtered before A/D conversion. In one embodiment, filtering canremove the undesirable effects of 60 cycle power-line related noiseexisting in the environment. Filtering can be added to the system.According to various embodiments, to improve the effective signal tonoise ratio (S/N). Care is taken to avoid eliminating the signal ofinterest (the picture element information), degrading resolution orsystem sensitivity. Output from filter 468 is communicated to block 490and can be stored in a buffer 471.

The scanner mirror position can be obtained from sensors incorporatedinto the scanner as described above in conjunction with FIG. 4B or othermethods of determining the position of the scanner can be employed.Optionally, a position of the scanner, which corresponds to the spot461, is communicated at 480 to block 490 and can be stored in the buffer471.

A measure of the intensity of the light source 452 is obtained, in oneembodiment, with a device 472 and a detector 476. In one embodiment, thedevice 472 is a partially transmissive mirror. The device 472 diverts aportion 474 of the first beam of light 454 to the detector 476. Thedetector 476 can be a PIN photodiode, avalanche photodiode (APD), aphotomultiplier tube (PMT), etc. In one embodiment, the output of thedetector 476 is converted to a digital signal at A/D converter 478. Thedigital signal output from the A/D converter 478 is communicated toblock 490 and can be stored in the buffer 471.

In one embodiment, the values in the buffer 471 corresponding to thescattered light energy from spot 461, the position of the scannercorresponding to spot 461 within the image, and the intensity of thelight source are stored in memory 480. In one embodiment, a dividefunction is applied to the data whereby the scattered light energy 462is normalized by the intensity of the light source obtained before thelight beam is scattered by the surface 460 to produce a correctedscattered-light-energy value corresponding to the light energy scatteredfrom the spot 461. The process is repeated for each spot on the surface460 corresponding to the collection of spots that compose the imagebeing acquired. Correction of the scattered light energy 462 accountsfor various non-idealities of the light source 452, such as a timevarying light source intensity. In various embodiments, the correctioncan be applied to a picture element (pixel), a group of pixels, a lineof an image, a frame of an image, etc. The present invention is notlimited by the discretization applied to the image.

An alternative embodiment to picking off a portion of the beam of light474 with device 472 and detector 476 is to place a reflector 494 (with aknown index of reflectance) such that the scanned beam 453 reflects offof reflector 494 and is detected by the detector 464. Often, thereflector 494 will be positioned outside of the field of view (FOV),though in some embodiments, the reflector may be within the FOV.

The configuration with the reflector 494 allows for a normalization ofthe image on a per line basis if the reflector 494 allowed the scannedbeam to be reflected as each line composing the image was scanned.Alternatively, an amount of the image acquired either before or after anintensity of the light beam is measured via reflector(s) 494 can benormalized at 490 by the measurement(s) of intensity. For example, iffour reflectors 494 are placed at intervals along a dimension of the(FOV), four measurements of the intensity can be made in the overscanregion corresponding to the location of the reflectors 494. Themeasurements of the intensity can be used to correct the scattered lightenergy collected from the surface 460. It will be recognized by those ofskill in the art that the reflector(s) 494 can be positioned anywhereoutside of the FOV, placement is not limited to the position shownwithin FIG. 4C. Alternatively, a light transmissive window definedbetween surfaces 458 a and 458 b may be made partially reflective and aportion of the beam energy may be picked-off substantially continuouslyfrom the partially reflective surface. AC-coupling of the scannedsurface 460 may be used to separate the substantially DC picked-offsignal corresponding to light source characteristics, or alternativelythe picked-off light may be directed to a separate detector.

Another alternative embodiment that eliminates device 472 from thesystem of 450 moves the detector 476 to location 492. Location 492 isany location outside of the FOV being scanned. As the scanned beam 453moves over the detector placed at location 492 a measure of theintensity of the beam is obtained before the beam is scattered by thesurface 460.

Any number of detectors 492 can be placed around the perimeter of theaperture formed by surfaces 458 a and 458 b or outside of the FOV.Detectors can be placed such that a measurement of the intensity of thescanned beam can be made with each line scanned or a detector can beplaced that allows one measurement of the intensity to be made with eachcomplete scan of the surface 460 (e.g., once per frame of image data).

FIG. 5 illustrates amplitude correction utilizing feedback to a lightsource drive according to one embodiment of the invention. Withreference to FIG. 5, a feedback system is illustrated generally at 500.A light source 502 has an optical output 504 a. The non-idealities,described above with respect to temporal amplitude fluctuations in theoptical intensity of a light source can be present in the output 504 aof the light source 502. In various embodiments, any of the lightsources described above can be utilized for light source 502. In oneembodiment, the light source is a DPSS laser. Amplitude fluctuations ofup to 30 percent can occur when a DPSS laser is used as a light source.

A device 506 is used to divert a known portion 508 of the incidentoptical beam 504 a to detector 510. In one embodiment, the device 506 isa partially reflecting mirror. The detector 510 can be any one of anumber of photoelectric devices that convert an optical signal into anelectrical signal. Such devices include, but are not limited to a PINphotodiode an avalanche photodiode (APD) and a photomultiplier tube(PMT). The electrical signal 512 is fed back to a compensation andcontrol stage 514. According to one embodiment, compensation and controlstage 514 adjusts the light source drive level to maintain a uniformlight source output 504 a by minimizing the diverted portion 508. In oneembodiment, the input signal used to drive amplifier 516 is adjusted bythe compensation and control stage 514. In one embodiment, the amplifier516 is the light source drive amplifier. In another embodiment, thelight source is driven with a periodic waveform having a frequency equalto or greater than pixel frequency, and light source compensation isrealized by modulating the duty cycle of the waveform. Other methods ofcontrolling the amplitude of the optical output of the light sourcebased on the diverted reference signal 508 and the compensation andcontrol stage 514 will be apparent to those of skill in the art based onthe teachings presented herein; embodiments of the present invention arenot limited thereby.

Compensation and control stage 514 continuously adjusts the light sourcedrive level to maintain a uniform optical output 504 b, which is used ina scanned beam image capture device such as those illustrated withinFIG. 4A and FIG. 4C. The non-idealities in the amplitude of the opticaloutput (amplitude noise) are substantially reduced by the feedbacksystem 500. Such a feedback system 500 can be used, in variousembodiments, in place of the correction methods and apparatusesdescribed in FIG. 4A and FIG. 4C or in addition to the correctionmethods and circuits described in those figures.

A non-ideality exhibited by some diode based light sources, such aslight emitting diodes (LEDs), is an optical output that drifts withtime. This characteristic is an undesirable non-ideality of the lightsource and can arise due to thermal drift, etc. In one or moreembodiments, a self-calibration of the light source is used to stabilizethe output of the light source; thereby substantially reducing theundesirable drift.

FIG. 6 depicts self-calibration of light source amplitude according toone embodiment of the invention, illustrated generally at 600. Withreference to FIG. 6, a light source 602 produces light 608 to be used indevices for capturing an image as described above in conjunction withFIG. 4A and FIG. 4C and for devices used to display images which aredescribed more completely below in the figures that follow. In oneembodiment, the light source 602 is driven by amplifier 604 at aconstant voltage; the output of the amplifier 604 can be interrupted byswitch (S1) 606. A reference current source 610 is connected throughswitch (S2) 612 to the input of the light source 602.

As described above, a device scans a beam across a surface to capture animage. In one embodiment, during the scanning process, the scanned beammoves in a raster pattern to illuminate the surface to be imaged with abeam of light. During the process of scanning, the scanned beam can bedirected into and out of the region to be imaged. The region that is notbeing imaged can be referred to as the overscan region. While thescanned beam is in the overscan region, a calibration is performed onthe light source 602.

In one embodiment, the process of self-calibration proceeds with a firstphase where a beam of light illuminates a surface to be scanned, eitherilluminating the surface with optical energy for the purpose ofproducing scattered optical energy from the surface or the beam of lightscans image data to be viewed as in a display device (described morefully below in conjunction with FIG. 9). During the first phase ofoperation, switch (S1) 606 is closed and switch (S2) 612 is open asindicated at 618 and 620 (first phase). During the second phase ofoperation, switch 606 opens and switch 612 closes, as indicated at 618and 620 (second phase). When switch 612 is closed, a reference currentis supplied to the input of the light source 602. A reference voltage614 is measured that corresponds to the reference current. In oneembodiment, the reference voltage is used at 616 to adjust the drivelevel at the input of the amplifier 604 to account for drift in thecurrent-voltage characteristic of the light source 602 during the nextperiod of operation.

Operation of the system continues with alternating first phase (imageacquisition or image display), and second phase (calibration); thereby,compensating for the non-ideality of a drifting light sourcecurrent-voltage characteristic. The calibration periods can be chosen tobe frequent, such as once every time a line of image data is eitheracquired or displayed, or can occur relatively infrequently as desired.Alternatively, the method can commence with the second phase(calibration) followed by the first phase (image acquisition or imagedisplay). In this case the calibration would occur first and the imageacquisition or display would occur second.

When the system 600 is implemented in a device used to scan a beam oflight for display to a user or viewer, the image signal to be displayedis input at 622. Scanned beam devices used to display image data to auser or viewer are described more fully below in the figures thatfollow.

FIG. 7A illustrates generally at 700, methods of controlling a beam oflight according to embodiments of the invention. With reference to FIG.7A, a light source 702 creates a beam of light. A variable window 704 isplaced in the optical path. The variable window has a time domainresponse that is tailored to provide a uniform output; the uniformoutput is indicated as corrected signal 706. The time domain response istailored to provide an inverse fluctuation with respect to fluctuationsin the input optical signal created by the light source 702. In oneembodiment, the variable window 704 is made using a photo opticpolymeric material. The photo optic material may function, for example,as a photo absorber to attenuate fluctuations in the amplitude of thelight beam created by the light source 702 that are above a desiredlevel, while allowing the optical energy to pass through when theamplitude is below the desired level.

In one embodiment, a variable optical Window is shown in FIG. 7B at 750.With reference to FIG. 7B, a light source 752 creates a beam of light754 that has an amplitude 756 that fluctuates as a function of time dueto various non-idealities as described above. A portion of the beam oflight is diverted by device 758 and is detected by detector 760. In oneor more embodiments, the detector 760 can be a PIN photodiode, an APD, aPMT, etc. which converts the optical energy into an electrical signal.In one embodiment, an analog to digital converter (not shown) convertsthe analog signal to a digital signal.

The output of the detector 760 is used as a reference to drive theoptical window 762. The optical window 762 has a time domain response766 as shown at 764, which provides an optical output 768 with amplitude770. The temporal fluctuations are removed from the output signal 768 bythe response of the optical window 762.

The optical window 762 can be constructed in a variety of ways. In oneembodiment, an electrically controlled window is constructed using aphoto chromic or electro chromic material. In one embodiment, theoptical window 762 is made using a liquid crystal material (LCD). Suchan optical window 762 corrects the temporal amplitude fluctuations inthe output of the light source 752 that occur in the 1-100 kilohertzrange. Other embodiments are readily adapted to address non-idealitiesoccurring in different frequency regimes within the light source output.

As described above, non-idealities in the properties of a light sourcewill lead to noise in the image acquired. In various embodiments, theeffects of noise on the image acquired are removed after the image isacquired by filtering the image. FIG. 8 illustrates generally at 800,removal of noise components from an image according to variousembodiments of the invention. With reference to FIG. 8, a method forremoving the effects of noise from the image acquired starts with someknowledge of the characteristics of the noise components and the imageat 802. An image is acquired at 804 and the effects of noise are removedat 806. Processing may then optionally continue at 808.

In various embodiments, the identification of noise components in imagedata can be performed in the time domain, where the relative phaseinformation exists between the different light sources or theidentification can be performed in image space after the image data hasbeen acquired.

In one or more embodiments, when three light sources are used to acquirea color image, such as a red, green, and blue (RGB) triad, it istypically the situation with most images that a noise component willshow up as a periodic phase relationship between any two of the lightsources, for example between the R and the B or between the R and G. Inone or more embodiments, if one light source is known to be stable, suchas the R, then the other, the B, and/or the G, is determined to havenoise on it.

In one or more embodiments, the image is captured and analyzed todetermine if any noise components are present that need to be removed.Most images produce RGB data that does not change disparately fast ordisparately slowly across the color channels R, G, and B. For example,when creating an image from a surface that includes two differentlycolored surfaces such as the flesh tone of a person and a wall of aroom, or when imaging across either one individually, one of the R, G,or B color channels generally does not have a real periodicity that doesnot show up at least at some level within the other color channels. Thatis, many real images are found to have correlation between variations inthe color channels. If a periodicity is detected in a given colorchannel that is not present in the other channels then, depending uponthe application, there may be a probability that the periodicity is anoise component and can be removed. On the other hand, if thefluctuation in a channel is constant from frame-to-frame within theimage, there is a probability that the fluctuation is a real imageattribute and the periodic fluctuation should not be removed from theoutput image.

In one embodiment, an example of such a periodicity in the image isamplitude noise on the output of a blue light source. In one example, aDPSS laser is known to exhibit amplitude noise in the band of 1-100kilohertz. A scanner such as the scanner described in conjunction withFIG. 4A through FIG. 4C is typically operated in a horizontal scandirection within the frequency band of 15 to 20 kilohertz. In oneembodiment operation of the scanner in the horizontal scan mode at 19kilohertz approximates Super Video Graphics Array (SVGA) graphicsresolution. Amplitude noise components present on the output of aparticular light source, such as the blue or green light source, cancreate undesirable periodic features in the image. Analysis of the colorchannels composing the image data is undertaken and the undesiredperiodic noise signal is removed from the image data.

Removal of noise components from the image data must be done with careso that actual image data is not removed in the process of removing theunwanted noise component. Indicators exist, which can be used to helpeliminate the removal of actual image data. For example, one indicatoris that the color channels may have variations on them that vary with aperiodicity corresponding to a sampling frequency. For example aperiodicity corresponding to a line scan time may be due to an edge orvariation in the image itself. Variations that may be related, throughimage analysis, to a contiguous set of points are generally not removedbecause of the probability they correspond to actual image features.Similarly, the probability is high that variations that occursimultaneously in all three color channels are related to the realimage, and therefore, these variations are not removed. On the otherhand, variations that occur randomly in a single color channel,especially with a channel having a light source known to be noisy,probably do relate to noise, and these variations are attenuated duringimage processing to eliminate the unwanted source noise. Thesecorrelations, as well as others, can be used to determine whether or notthe suspect signal should or should not be removed from the image.

FIG. 9 shows generally at 900, correction of light source non-idealitiesin a device used to display an image according to one embodiment of theinvention. With reference to FIG. 9, a scanned beam display 902 receivesa source of an image(s), such as a signal 904 a, which in oneembodiment, will be scanned onto the retina of a viewer's eye 914. Whilethe system as presented in FIG. 9, scans light containing image dataonto the viewer's eye 914, the structures and concepts presented hereincan be applied to other types of displays, such as projection displaysthat include viewing screens, etc.

Control electronics 920 provide electrical signals that controloperation of the display 902 in response to the signal 904 a. Signal 904a can originate from a source such as a computer, a television receiver,videocassette player, DVD player, remote sensor, or similar device. Inone embodiment, a similar device is an imaging sensor in a digitalcamera or a digital video camera, etc.

The light source(s) 906 outputs a modulated light beam(s) 908. The lightbeam(s) has a modulation which corresponds to information in the imagesignal. In one embodiment, light source 906 is a triad of light sourcescomprising the colors red, green, and blue (RGB). In other embodiments,the light sources can include more or less than three individual lightsources, additionally the light sources can utilize colors other than orin addition to the colors red, green, and blue. The light source 906 canutilize a coherent light source such as a laser diode, a diode pumpedsolid state (DPSS) laser, a laser, etc. The light source 906 can alsoutilize a non-coherent source such as a light emitting diode (LED). Thelight source 906 can include directly modulated light emitters such asthe LEDs or may use continuous light emitters indirectly modulated by anexternal modulator such as an acousto-optic modulator.

A scanner 910 deflects the modulated light beam 908 to produce scannedbeam 912 which is scanned onto the retina of the viewer's eye 914. Thescanner 910 is typically a bidirectional scanner that scans in both thehorizontal and vertical directions to produce the image. In variousembodiments, the scanner 910 has been described in preceding figures.

In an alternative embodiment, an optional lens 924 is included; the lens924 is formed from a curved partially transmissive mirror that shapesand focuses the scanned beam 912 for viewing by the viewer's eye 926.Because the lens 924 is partially transmissive, the lens 924 combinesthe light from the scanner 910 with the light received from thebackground 928 to produce a combined input to the viewer's eye 926.Although the background 928 presented here is a “real-world” background(tree) the background light can be occluded as it is when the display isviewed at 914. One skilled in the art will recognize that a variety ofother structures may replace or supplement the lenses and structuresshown in FIG. 9. For example, a diffractive element such as a Fresnellens may replace the lens 924. Alternatively, a beamsplitter and lensmay replace the partially transmissive mirror structure of the lens 924.Other optical elements, such as polarizing filters, color filters, exitpupil expanders, chromatic correction elements, eye tracking elements,and background masks may also be incorporated for certain applications.

In one embodiment, various non-idealities in the amplitude of the outputof the light source 906 can be corrected with the system 600 describedin FIG. 6. Control electronics 920 together with the light source 906can be operated to perform a self-calibration in the overscan regionsduring display of the image data for viewing by a viewer at 914 or 926.Similar to the image capture device described above in the precedingfigures, the scanned beam 912 can be made to travel through a firstregion, such as the field of view (FOV) and into a second region, anoverscan region, where illumination of the light source is not visibleto the viewer. In the overscan region, a reference signal is applied tothe light source. A parameter of the light source is measured inresponse to the reference current, and the light source drive isadjusted as a function of the measured parameter. In one embodiment, thelight source is a light emitting diode (LED), the reference signal is areference current and the parameter that is measured is the forwardvoltage on the LED.

In another embodiment, a shape of a spectral power distribution (SPD) ofenergy output from an InGaN-based blue or green edge emitting lightemitting diode (EELED) changes as a function of drive level. The drivelevel dependence causes a shift in a dominant wavelength of the lightsource that is generally known a priori. To compensate for such anon-ideality, a transformation is performed. The transformation is basedon the color information within signal 904 a and the known shift inlight source dominant wavelength that will occur if the signal 904 a isdisplayed without correction. The properties of color contained in thesignal 904 a can vary according to the application; however, in oneembodiment the color information includes information on hue,saturation, and luminance. In one embodiment, on a picture element(pixel) basis, the transformation transforms the white point of a colorgamut while maintaining a constant luminance between the signal 904 aand the luminance that will result subsequent to modulation of the lightsources, such that the signal 904 b can be displayed throughout therange of light modulation levels required to display the image data.

In one embodiment, a lookup table 923 is stored in memory 922. Thelookup table 923 facilitates transformation of the picture elementinformation; thereby, correcting for the shift in white balanceresulting from the drive dependent shape of the spectral powerdistribution (SPD) of the EELED light sources. In one embodiment, afurther description of a set of algorithms used to generate the lookuptable 923 is provided in conjunction with FIG. 14 through FIG. 19 below.

The shift in SPD as a function of drive level, previously described, isan example of a light source non-ideality that can be known a priori andcan be compensated for down to the picture element (pixel) level withthe methods described above, such as using a lookup table or performingother data processing as is appropriate.

In another embodiment, when a non-ideality remains relatively constantfor a time period that lasts longer than a pixel duration (a pixelduration is typically 18 nanoseconds) the effects of the non-idealitycan be corrected by obtaining information on the dominant wavelength ofthe light source and then using the information to perform the necessarycorrection to the image data at 920. In some embodiments, a non-idealityoccurs (which can be substantially constant over time) when a lightsource is used to either capture an image or to display an image and thelight source has a dominant wavelength that produces a color gamut (whenused in conjunction with additional light sources) that is offset from acolor gamut that would have been produced if the light source had adifferent dominant wavelength. Such non-idealities are corrected invarious embodiments as described below.

In one embodiment, a device 930 diverts a portion 932 of the output ofthe light source 906 to a device 934. In one embodiment, the device 930is a partially transmissive mirror. The device 934 is a device thatprovides information on the dominant wavelength of the light source andin some embodiments a measure of a spectral power distribution (SPD) ofthe light source such as a spectrometer. For cases where a SPD iscompensated for, the term detector should be taken to include SPDdetecting devices. A spectral power distribution of a light source andthe associated dominant wavelength are described below in conjunctionwith FIG. 12. It will be recognized by those of skill in the art that adominant wavelength of a light source provides information on a SPD ofthe light source; the dominant wavelength of the light source is theportion of the SPD where a majority of the spectral power resides. Forthe purpose of this description, the terms “dominant wavelength” or“information on a dominant wavelength” of a light source will beconsidered to include information on a SPD of the light source.Information on the dominant wavelength of the light source 906 is outputfrom the device 934 and is input at 936 to control electronics 920.

The control electronics 920 utilize the measured information on thedominant wavelength(s) of the light source(s) to perform the colortransformation and adjustment to the modulation of the light sources tocompensate for the shift in dominant wavelength of the light source(s).Image information, input at 904 a, contains color information for thepicture elements (pixels) that compose the image or images. Theproperties of color contained in such images can vary according to theapplication. For example, discussion above referred to RGB values. Inother embodiments, such values may be transformed to hue, saturation,and luminance. In one embodiment, a transformation is applied to thesignal 904 a to transform the color associated with signal 904 a toaccount for a shift in the dominant wavelength of a light source(s),resulting in signal 904 b. In another embodiment, a transformation isapplied to the signal 904 a to account for a system 900 that is madeusing light sources that produce a color gamut other than the colorgamut of interest. Such transformations are performed by taking thecolor information of the picture element or elements (904 a) andtransforming that information using a mathematical model to describecolor to produce signal 904 b. Various models for color andtransformation of color are described more fully below in conjunctionwith FIG. 14 through FIG. 19.

In various embodiments, the methods and apparatuses described herein areused to correct non-idealities of component or subsystem properties.These methods and apparatuses can be used in implementations of systemsthat correct for various characteristics of scanned beam devicesdescribed herein.

FIG. 10 is a block diagram of a first system for generating an outputclock to retrieve data from a memory matrix while compensating fornonlinear scan speed of the resonant mirror, according to one embodimentof the invention. In the embodiment shown in FIG. 10, a corrected clockis produced from a pattern generator rather than a counter to controlclocking of output data. A synch signal stripper 1000 strips thehorizontal synchronization signal from an arriving image signal V_(IM).Responsive to the synch signal, a phase locked loop 1002 produces aseries of clock pulses that are locked to the synch signal. An A/Dconverter 1004, driven by the clock pulses, samples the video portion ofthe image signal to produce sampled input data. The sampling rate willdepend upon the required resolution of the system. In the preferredembodiment, the sampling rate is approximately 40 MHz. A programmablegate array 1006 conditions the data from the A/D converter 1004 toproduce a set of image data that are stored in a buffer 1008. Oneskilled in the art will recognize that, for each horizontal synchsignal, the buffer will receive one line of image data. For a 1480×1024pixel display, the system will sample and store 1480 sets of image dataduring a single period of the video signal.

Once each line of data is stored in the buffer 1008, the buffer isclocked to output the data to a RAMDAC 1009 that includes a gammacorrection memory 1010 containing corrected data. Instead of using thebuffer data as a data input to the gamma correction memory 1010, thebuffer data is used to produce addressing data to retrieve the correcteddata from the gamma correction memory 1010. For example, a set of imagedata corresponding to a selected image intensity I1 identifies acorresponding location in the gamma correction memory 1010. Rather thanoutput the actual image data, the gamma correction memory 1010 outputs aset of corrected data that will produce the proper light intensity atthe user's eye. The corrected data is determined analytically andempirically by characterizing the overall scanning system, including thetransmissivity of various components, the intensity versus currentresponse of the light source, diffractive and aperture effects of thecomponents and a variety of other system characteristics.

FIG. 11 is a block diagram of an alternative embodiment of the apparatusof the block diagram of FIG. 10. In one embodiment shown in FIG. 11according to the invention, the data may be corrected fortemperature-versus-intensity or age-versus-intensity variations of thelight source. Reference data drives the light source. For example, atthe edge of the horizontal scan, the reference data is set to apredetermined light intensity. A detector 1019 monitors the power out ofthe light source 1016 and optionally a temperature compensation circuit1021. If the intensity is higher than the predetermined light intensity,a gain circuit 1023 scales the signal from the RAMDAC 1009 by acorrection factor that is less than one. If the intensity is higher thanthe predetermined light intensity, the correction factor is greater thanone. While the embodiments described herein pick off a portion of theunmodulated beam or sample the beam during non-display portions of thescanning period, the invention is not so limited. For example, a portionof the modulated beam can be picked-off during the display portion ofthe scanning period or continuously. The intensity of the picked-offportion of the modulated beam is then scaled and compared to the inputvideo signal to determine shifts in the relative intensity of thedisplayed light versus the desired level of the displayed light tomonitor variations.

In addition to monitoring the intensity, the system can also compensatefor pattern dependent heating through the same correction data or bymultiplying by a second correction factor. For example, where thedisplayed pattern includes a large area of high light intensity, thelight source temperature will increase due to the extended period ofhigh level activation. Because data corresponding to the image signal isstored in a buffer, the data is available prior to the actual activationof the light source 1016. Accordingly, the system can “look-ahead” topredict the amount of heating produced by the pattern. For example, ifthe light source will be highly activated for the 50 pixels precedingthe target pixel, the system can predict an approximate patterndependent heat effect. The correction factor can then be calculatedbased upon the predicted pattern dependent heating. Although thecorrection has been described herein for the intensity generally, thecorrection in many embodiments can be applied independently for red,green and blue wavelengths to compensate for different responses of theemitters and for variations in pattern colors. Compensating for eachwavelength independently can help limit color imbalance due to differingvariations in the signal to intensity responses of the light emitters.Alternatively, compensation may be performed on fewer than allwavelengths.

Returning to FIG. 10, the corrected data output from the gammacorrection memory 1010 (as it may be modified for intensity variations)drives a signal shaping circuit 1014 that amplifies and processes thecorrected analog signal to produce an input signal to a light source1016. In response, the light source 1016 outputs light modulatedaccording to the corrected data from the gamma correction memory 1010.The modulated light enters a scanner 1018 to produce scanned, modulatedlight for viewing.

As described above, non-idealities in light source parameters can causea shift in a spectral power distribution of a light source and acorresponding change in a dominant wavelength of the light source. FIG.12 shows, generally at 1200, a shift in a dominant wavelength of anInGaN-based green edge emitting light emitting diode (EELED) at twodifferent drive levels. With reference to FIG. 12, a spectral powerdistribution 1202 is plotted for the InGaN-based green EELED. Thehorizontal axis 1206 corresponds to wavelength and the vertical axis1204 corresponds to magnitude of the spectral power distributionsplotted thereon. A peak 1208 of the SPD 1202 corresponds to a dominantwavelength of the light source at a first drive level d₁.

In one embodiment, during modulation of the light source, the drivelevel is changed to d₂. A new spectral power distribution (SPD) 1210results at drive level d₂. PSD 1210 has a corresponding dominantwavelength 1212. It will be observed from FIG. 12 that 1208 correspondsto a wavelength of 520 nanometers and 1212 corresponds to 500nanometers.

In one embodiment, an InGaN-based blue light source can be used alongwith a red light source to display image data; such a triad of lightsources is capable of producing a color gamut. Other numbers of lightsources can be used to produce a color gamut, such as four, five, six,etc. Various constructs have been developed to describe color. Onesystem used frequently with electronic display systems is RGB, whichcorrelates spectral distribution and intensity to the output ofconventional three-channel display systems. Another system; hue,saturation, and luminance; may be especially useful in transformationsthat require linearly independent variables. Constructs include, theCommission Internationale de I'Eclairage (CIE) system, the Munsell ColorSystem, the Ostwald Color System, the Newton Color Circle, etc. In oneembodiment, a method of transforming image data from a first color gamutto a second color gamut is described utilizing the CIE system; however,other color systems can be used as well as empirical methods,embodiments of the present invention are not limited to transformationsusing one particular system.

FIG. 13 shows, generally at 1300, a shift of a color gamut for a deviceutilizing a red light source, and a green and blue EELED as lightsources. With reference to FIG. 13, the CIE color space is shown withthe x chromaticity coordinate plotted at 1304 and the y chromaticitycoordinate plotted at 1302. The chromaticity coordinates x and y map allcolors perceivable to the human eye within a curve defined by 1306. Thespectral colors that are within the visible color spectrum are foundalong the curve 1306.

In one embodiment, operation of three light sources is defined by thepoints 1308,1310, and 1312 and creates a first color gamut indicated bythe area within the triangular region, defined by the dashed line. Thecolor gamut has a white point at 1322. Point 1310 corresponds to drivelevel d₁ (FIG. 12) and the corresponding dominant wavelength 1208 (FIG.12) at 520 nanometers. Drive level d₂, (FIG. 12) results in a shift ofthe dominant wavelength of the light source to 1212 (FIG. 12) at 500nanometers, which creates a new color gamut defined by the triangularregion prescribed by points 1308, 1314, and 1316 (FIG. 13). In theembodiment illustrated in FIG. 13, the dominant wavelength of the bluelight source has also shifted from 1312 at a first drive level to 1316at a second drive level. The new color gamut has a white point 1324,which is different from the white point of the first color gamut 1322.

Various other color gamuts (and the accompanying shift in white balance)will result as the light sources are modulated over a range of drivelevels which are necessary to display the image data. As an example, thegreen light source will trace a path along curve 1307 as its drive levelis incremented from d₁ to d₂. Such intermediate locations are indicatedfor illustration purposes only as points 1318; the movement is indicatedby arrow 1320. Such a situation of shifting color gamuts may produceundesirable results in the displayed image. In one embodiment, theseundesirable results are compensated for by transforming colors from onecolor gamut to another.

FIG. 14 is a flow chart 1400 of a technique used to perform colorcorrection according to one embodiment of the invention. With referenceto FIG. 14, image data (pixels) are transformed to provide proper whitebalance and color rendering over a range of luminance levels. Thetechnique begins at block 1402, where picture element information(pixel) is input in the form of normalized RGB (red, green, blue) data,whose values span a range of 0 to 1 and which are characterized by awhite point, R=G=B=1. In one embodiment, the white point is the D6500white point.

Block 1404 maps an input RGB value to an R′G′B′ value. The R′G′B′ valuemaintains the total luminance of the RGB value while adjusting thecombined chromaticity coordinates to match the white point of the RGBvalue. In one embodiment the mapping from RGB to R′G′B′ is done with alookup table. The lookup table can be generated in a variety of wayssuch as empirically or by using known (measured) EELED dominantwavelength/luminance dependencies and an iterative search algorithmdescribed below.

A correction factor is calculated at block 1406, where the k_(r), k_(g),and k_(b) values are equal to scaling factors chosen to generate adesired white point at full luminance for a given system. A transformedR″G″B″ value is obtained at block 1408. In one embodiment, thetransformation of a general RGB color value (that is not the white pointor a spectral color) is done by interpolating between the white pointand the spectral colors.

FIG. 15 illustrates correction of the white point of a color gamut, as afunction of luminance level, according to one embodiment of theinvention. With reference to FIG. 15, table 1502 contains atransformation of a white point of a color gamut for 10 differentluminance levels listed at 1504. For a given row in table 1502, an RGBpoint is shown at 1506 and the corresponding transformed R′G′B′ point isshown at 1508. The table 1502 can be generated at any increment inluminance; thereby, producing a general number of rows 1510.

The data displayed in table 1502 is plotted in a graph 1520. Thehorizontal axis 1522 of the graph 1520 is labeled “Nominal RGB values”and the vertical axis 1524 is labeled “Corrected RGB values,” (whichcorrespond to the R′G′B′ values. The “Corrected RGB values” (from 1508)are plotted against the values from 1506. For the particular systemrepresented by the lookup table of FIG. 15, it is observed that there isa relative increase in luminance for the blue (B′) values 1530 and arelative decrease in the luminance for the red (R′) values 1526. Thecorrected green (G′) values 1528 are essentially equivalent to thenominal values 1532. In other embodiments, the lookup tables will bedifferent and the relative trends will be different. The data presentedin FIG. 15 is merely for illustrative purposes and does not limit theembodiments of the invention within this description.

Several additional techniques may be used with the technique describedabove in conjunction with FIG. 14. FIG. 16 is a block diagram for atechnique used to generate the lookup table of FIG. 15, according to oneembodiment of the invention. With reference to FIG. 16, an initialluminance point is set at block 1602, such as R=G=B=0.5.

The RGB values are corrected to the desired white point whilemaintaining the initial luminance at block 1604. In one embodiment, thewhite point is D6500. It will be recognized by those of skill in the artthat other white points can be maintained, D6500 has been selected forillustration only. The technique implemented within block 1604 isdescribed below in conjunction with FIG. 17. The technique executed inblock 1604 determines R′G′B′ values for 1508 (FIG. 15) and builds thelookup table at block 1606. At block 1608 the process continues byincrementing the luminance values and proceeding to repeat 1610 theblocks again at 1602 with the incremented luminance values.

FIG. 17 illustrates correction of the white point of a color gamut at afixed luminance level, according to one embodiment of the invention.Block 1604 (FIG. 16) represents the method depicted in FIG. 17. Withreference to FIG. 17, the x, and y CIE chromaticity color coordinatesare determined for each EELED 1702 as a function of luminance level,utilizing measurements of dominant wavelength and peak spectralintensity (as functions of EELED drive level). In the embodimentdescribed, the red light source maintained a constant spectral outputwith drive level. In other embodiments, a red light source might exhibitnon-idealities that would require correcting; the present invention isnot limited to the embodiment presented in this description. At block1704, the white point (coordinates) of the light sources at a givenluminance value is computed using the algorithm described below inconjunction with FIG. 18. At block 1706, the white point calculated atblock 1704 is compared to the desired white point. If the white pointcalculated at block 1704 is different from the desired white point, themethod of FIG. 19 is called at block 1706 (FIG. 17) to adjust the RGBvalues until the desired white point has been achieved. When the desiredwhite point has been achieved, the adjusted RGB values corresponding tothe desired white point become the R′G′B′ values 1508 of the lookuptable 1502 (FIG. 15).

FIG. 18 illustrates an algorithm to generate CIE chromaticity colorsfrom RGB values, according to one embodiment of the invention. Withreference to FIG. 18, at block 1802 the luminances are calculated fromthe RGB values of the three light sources. At block 1804 the Xtristimulus value is computed from the luminances and the x, y colorcoordinates for each light source, where xr, xg, and xb indicated the xchromaticity coordinate of the red, green, and blue light sourcesrespectively. Similarly, yr, yg, and yb indicate the y chromaticitycoordinate of the red, green, and blue light sources respectively. Atblock 1806 the Z tristimulus value is computed from the luminances andthe x, y color coordinates for each light source. At block 1808 the Ytristimulus value is computed as the combined luminous from each of thelight sources. At block 1810 the combined x, and y CIE color coordinatesare determined from X, Y, and Z tristimulus values.

FIG. 19 illustrates a method to adjust a corrected white point whilemaintaining constant luminance, according to one embodiment of theinvention. The method of FIG. 19 is called at block 1706 (FIG. 17) toadjust the RGB values until the desired white point has been achieved.With reference to FIG. 19 at block 1902, if the calculated white point(from FIG. 18) is too blue, R is incremented and B is decremented, whilemaintaining constant luminance. Such a condition is indicated when the xcoordinate, of the white point, is less than the x coordinate, of thedesired white point, such as 0.3127 in the case of D6500 white). Aninterval for incrementing and decrementing, ΔR, ΔG, and ΔB, and therelationships needed to maintain constant luminance, are indicated at1910, where in one embodiment INCREMENT=0.0001.

At block 1904, if the calculated white point is too red, R isdecremented and B is incremented, while maintaining constant luminance.Such a condition is indicated if the x coordinate, of the white point,is less than the x coordinate, of the desired white point, such as0.3127 in the case of D6500 white.

At block 1906, if the calculated white point is lacking green, R isdecremented and B is decremented, while maintaining constant luminance.Such a condition is indicated if the y coordinate, of the white point,is less than the y coordinate, of the desired white point, such as0.3291 in the case of D6500 white.

At block 1908, if the calculated white point is too green, R isincremented and B is incremented, and G is decremented while maintainingconstant luminance. Such a condition is indicated if the y coordinate,of the white, point is greater than the y coordinate, of the desiredwhite point, such as 0.3291 in the case of D6500 white.

The methods and transformations described above in the preceding figuresare representative of various embodiments of the invention. Othermethods and transformations can be implemented to achieve correction ofshifts in a dominant wavelength of one or more light sources used indisplays and scanned beam devices. For example, in other embodiments,other color models can be used and other methods can be employed tocompensate for the light source non-idealities.

In one or more embodiments, a gamma correction can be used to correctfor various non-idealities of light sources, such as a shift in adominant wavelength of a light source with modulation level. In oneembodiment, the gamma correction is applied to a light emitting diode(LED) light source.

In one embodiment, FIG. 20 illustrates a method to correct changes in awhite balance of a display, utilizing gamma correction. With referenceto FIG. 20, the method begins at block 2002 where luminances for a setof light sources, used in the display, are set to maximum values and thedesired white point is also set using the maximum luminance levels forthe light sources. The processes described in blocks 2004, 2006, and2008 are performed on each light source individually until all of thelight sources have been tested. The process of FIG. 20 can be performedon polychromatic devices or on monochromatic devices, as well as lightsources that can be directly modulated, such as light emitting diodes(LEDs) and on light sources that require external modulation.

At block 2004 a light source, indicated by X, is modulated across arange of luminances, typically the range of luminances will be the fullrange of luminances within the modulation range of the light sources. Inone embodiment, three light sources are used; therefore, X will be takenfrom the set [R,G,B]. In one embodiment, the luminances for a set oflight sources are referred to by device code, where device code rangesfrom 0, to 255.

At block 2006 the luminance intensity is measured across the range ofluminance values. In one embodiment, the luminance intensity and the CIEcolor coordinates are measured with an Ocean Optics, Inc. S2000Miniature Fiber Optic Spectrometer.

At block 2010 the luminance intensity is analyzed and the measuredluminance intensity is corrected. In one embodiment, the correction foreach light source is done with a look up table (LUT) where code mapsonto itself. In the LUT, the old code is mapped to a new code such thatthe new code maps to an intensity that follows the gamma relation shownat block 2012. Knowing the desired gamma relation facilitates using asort algorithm on the measured data or an inverse relation to find thecorresponding code that gives the desired gamma relation. In oneembodiment, separate gammas are chosen such that a color luminance ratiofor each light source at a given code value maintains the white pointselected in block 2002. This is possible, because the color coordinateswere measured for the old code and can be used to predict the colorcoordinates of the new code. To determine if the correct color luminanceratios were chosen, a check is made by measuring the CIE colorcoordinates at intermediate code values with a spectrometer or colorchromaticity meter. If the luminance ratio of the light sources isincorrect, an adjustment is made by selecting new gammas to maintain thewhite point selected in block 2002. Such a correction can, in somecases, result in the individual RGB gamma corrections crossing eachother as shown in FIG. 21. As described above, the luminance intensity,L_(X), follows the functional relationship shown at block 2012.

The corrections are used to correct the image data before the lightsources are modulated. Following the method described above, separategamma corrections are obtained for each light source in the display.Each gamma correction is a function of the modulation level of the lightsources. In one or more embodiments, a modulation level is equivalentwith a light source drive level. In various embodiments, the drivelevels of LEDs are modulated to display image data on a display.Individual gamma corrections for each LED composing a display areincorporated into the display to correct the non-idealities that occurfrom drive level dependent shifts in the dominant wavelength of theLEDs.

FIG. 21 shows individual RGB gamma corrections, according to the methodof FIG. 20, for one embodiment of the invention. With reference to FIG.21, luminance intensity is plotted as a function of code value for threeLED light sources in a display. The vertical axis 2104 corresponds toluminous intensity and the horizontal axis 2102 corresponds to codevalue. The gamma correction for the red LED is plotted as curve 2108.The gamma correction for the green LED is plotted as curve 2106 and thegamma correction for the blue LED is plotted as curve 2110.

The method of FIG. 20 has been applied, in one embodiment, to a seriesof LEDs in FIG. 21; however, the method can be applied to the modulationof an externally modulated device, such as a laser based device.Alternatively the method of FIG. 20 can be applied to variouscombinations of directly modulated light sources, such as LEDs, andexternally modulated light sources, such as laser based light sources.In one or more embodiments, a device can include one or more directlymodulated light sources and one or more externally modulated lightsources.

FIG. 22 illustrates correcting effects of spectral power distribution(SPD) variations, according to one embodiment of the invention. Withreference to FIG. 22, a device capable of capturing an image is showngenerally at 2200. A light source 2202 creates a first beam of light2204. A scanner 2206, which includes a mirror, deflects the first beamof light 2204 to produce a scanned beam 2203, the scanned beam 2203passes through an aperture defined by surfaces 2208 a and 2208 b toilluminate a spot 2211 on a surface 2210. The scanner 2206 has beendescribed above in the preceding figures.

While the scanned beam 2203 illuminates the spot 2211 the scanned beam2203 is scattered from the surface 2210 at the spot 2211 to producescattered light energy 2212. The scattered light energy 2212 is receivedby one or more detectors 2242 and is converted into a digital signal atanalog to digital (A/D) converter 2244. Different types of detectors canbe used for the detector 2242. In one embodiment, the detector 2242 is apositive-intrinsic-negative (PIN) photodiode. In the case ofmulti-colored imaging, the detector 2242 can include splitting andfiltering functions to separate the scattered light into component partsbefore detection. Other detectors can be used, such as but not limitedto, an avalanche photodiode, (APD) or a photomultiplier tube (PMT). Insome embodiments, the detector 2242 collects light through filters (notshown) to reduce ambient light. In various embodiments, the detector(s)can be arranged to stare at the entire field of view (FOV), a portion ofthe FOV, collect light retrocollectively, or collect light confocally.

The output of the detector 2242 is received eventually at block 2232.Appropriate filtering can be applied to the signal at 2246. Optionally,a position of the scanner 2206, which corresponds to the location of thespot 2211, is communicated at 2230 to block 2232.

Information on the SPD of the light source 2202 is obtained, in oneembodiment, with a device 2224. As described above, information on theSPD of a light source includes information on a dominant wavelength ofthe light source. Examples of such information include, but are notlimited to, an integral of a SPD, a measure of a dominant wavelength, anincrease in a dominant wavelength, a decrease in a dominant wavelength,etc. Such information, together with knowledge of an initial dominantwavelength of a light source is used to correct the scattered lightenergy 2212. A device 2222 diverts a portion of the first beam of light2204 to the device 2224. In one embodiment, the device 2222 is apartially transmissive mirror. The device 2224 is described more fullybelow in conjunction with FIG. 23. The output of the device 2224 iseventually received at block 2232. The output of the device 2224 may beconverted to a digital signal by A/D converter 2226.

In one embodiment, the scattered light energy 2212 is corrected by theinformation on the SPD of the light source 2202 to produce a correctedscattered-light-energy value corresponding to the light energy scatteredfrom the spot 2211. The process is repeated for each spot on the surface2210 corresponding to the collection of spots that compose the image ofthe surface 2210. Correction of the scattered light energy 2212 accountsfor various non-idealities of the light source 2202, such as a timevarying dominant wavelength of the SPD of the light source. In variousembodiments, the correction can be applied to a picture element (pixel),a group of pixels, a line of an image, a frame of an image, etc. Thepresent invention is not limited by the discretization applied to theimage.

In one embodiment, a correction is applied to the scattered light energy2212, which remaps the red, green, and blue (RGB) values of the pictureelement information (pixels) based on the measured SPD information, suchas the actual dominant wavelength of the light source 2202, a shift inthe dominant wavelength, etc. Remapping RGB values can be performed inone embodiment according to the method described above in conjunctionwith FIG. 14 through FIG. 19. The correction, in various embodiments, isperformed by processing or manipulating the data representing thescattered light energy 2212. As will be recognized by those of skill inthe art, such processing can be performed in the block 2232 by softwareor hardware or a combination of both.

In one embodiment, block 2232 contains a buffer 2234. The buffer 2234receives data representing the scattered light energy 2212, the measuredSPD information, and optionally the position 2230 of the scanner 2206.Block 2232 can include memory 2236, which can store one or more framesof image data. The lookup table 2240 can be used to perform theremapping 2238 of the RGB values, described above, resulting incorrected image data 2270.

In one embodiment, the functions of the device 2224 can be performed bya device 2260 and the detector 2242. In this embodiment, the scannedbeam 2203 a illuminates the device 2260 in an overscan region that isnot in a field of view (FOV) (the FOV is a region where scattering froma surface is being measured). Generally, the device 2260 can be locatedanywhere along the optical path that is free of unquantified scatteringsources. In one embodiment, the device 2260 can be located on thesurface 2210. A scattered beam 2212 a can be received by a detector2242; thereby, obtaining information of the spectral power distributionof the light source 2202.

According to various embodiments, the image capture device 2200 can beused in a variety of applications, such as but not limited to, a digitalcamera, a bar code reader, multidimensional symbol reader, documentscanner, or other image capture or acquisition device. To allow thedevice to gather light efficiently, the device 2200 can includegathering optics (not shown) that collect and transmit light from thesurface 2210 to the device 2200. The gathering optics are configured tohave a depth of field, a focal length, a field of view (FOV), and otheroptical characteristics appropriate for the particular application. Forexample, in one embodiment, where the device 2200 is a two-dimensionalsymbology reader, the gathering optics may be optimized for red orinfrared light and the focal length may be on the order of 10-50centimeters. For reading symbols at a greater distance, the focusingoptics may have a longer focusing distance or may have a variable focus.The optics may be positioned at various locations along the optical pathto allow smaller, cheaper components to be used.

FIG. 23 illustrates various devices 2300, used to obtain information onthe spectral power distribution (SPD) of a light source, according toembodiments of the invention. With reference to FIG. 23, in oneembodiment, a diffractive optical element 2304 is in a path of anoptical beam 2302. The angle 2308 will be a function of the dominantwavelength of the optical beam 2302. A detector arranged to sense theangle 2308 or changes in the angle 2308 will provide information on thedominant wavelength of the optical beam. In one embodiment, thediffractive optical element 2304 can direct the beam 2306 to aphoto-resistor 2310 that is part of an arm 2312 of a Wheatstone bridge.Variations in the output of the photo-resistor 2310 can be related tothe dominant wavelength of the optical beam through the diffractionangle 2308.

In another embodiment, a first photodetector 2314 and a secondphotodetector 2316 are positioned to be illuminated by an optical beam2318. The relative output of the photodetectors is calibrated todominant wavelength shift in the optical beam 2302; thereby, obtaininginformation on the dominant wavelength of the optical beam 2302. Inanother embodiment, an array of photodetectors 2320 containsphotodetectors 2322. The optical beam 2306 illuminates the array 2320.The array 2320 is used in various configurations to provide a measure ofthe dominant wavelength within the optical beam by calibrating theoutput of the array 2320 to the dominant wavelength of the optical beam.

In one embodiment a response curve of a photodetector is shown at 2334,where the vertical axis 2330 corresponds to amplitude out of thedetector and the horizontal axis 2332 corresponds to wavelength of theoptical energy incident upon the detector. Typically, photodetectorsexhibit a response that has a less sloped portion, indicated at 2335 anda sloped portion indicated at, for example, a measurement range 2336.The optical beam 2302 can be illuminated directly on a photodetectorselected to have its sloped response 2336 in the range of dominantwavelengths of interest. Such a configuration does not require thediffractive element 2304. The output of a photodetector so configuredcan be calibrated to the dominant wavelength of the optical beamincident thereon.

In one embodiment, a detector is configured to place a firstphotodetector 2314 and a second photodetector 2316 in the path of anoptical beam. The pair of photodetectors is used to measure both theamplitude of the energy in the optical beam and the dominant wavelengthof the optical beam. In one configuration, the first photodetector 2314is selected to place a portion of its response 2335 within a range ofdominant wavelengths appropriate for the optical beam. The secondphotodetector 2316 is selected to place its sloped response 2336 withinthe same range of dominant wavelengths. Such a pair of photodetectors,so selected, provides a measure of both the amplitude and the dominantwavelength of the light source used to create the optical beam. Themeasure of the amplitude (made with the first photodetector) can be usedto correct non-idealities in the amplitude of the light source and themeasure of the dominant wavelength (made with the second photodetector)can be used to correct non-idealities in the dominant wavelength of thelight source. In this dual photodetector configuration a diffractiveelement is not required.

In another embodiment, an object 2340 having a known spectralreflectance is placed in the optical path to obtain a scattered signal(received by a detector) that provides information on the spectral powerdistribution of the light source. In one embodiment, such an object 2340is a white spot. An optical beam, which can be the output of a coloredlight source, is reflected from the white spot and the reflected signalis processed knowing the reflectance of the white spot to determinedominant wavelength of the light source.

In various embodiments, the object 2340 having a known spectralreflectance can be colored paint or colored ink, a wavelength selectivereflector, stacked reflectors, such as stacked quarter-wave reflectors,a certain material like a metal, etc. The object 2340 can be any objectwith a known spectral reflectance, the present invention is not limitedby the object 2340 used to obtain the known scattered energy.Alternatively, a spectrometer could be used to obtain information on thespectral power distribution of the light source.

The devices described above in conjunction with FIG. 23 can be used inthe description of the systems herein, used to capture and to displayimages, such as detector 2242, detector 2250, device 2260, device 2224(FIG. 22), etc. and other figures above.

Another non-ideality that exists in a system used to capture or displayan image is the use of a light source that creates a non-ideal colorgamut. In such a case, it is desirable to transform the color gamutresulting from the actual colors of the light source to another colorgamut.

For example, referring back to FIG. 13, a first color gamut defined bythe triangular region prescribed by points 1308, 1314, and 1316represents, in one embodiment, the actual color gamut of a system. Asecond color gamut defined by the triangular region prescribed by points1308, 1310, and 1312 represents, in one embodiment, the preferred colorgamut of the system. However, the system is operated with light sourcesthat produce colors in the first color gamut. To obtain colors from thesystem in the second color gamut a transformation is made between thefirst color gamut and the second color gamut. For the purpose of thisdescription the CIE color system has been used to describe a colorgamut, embodiments of the present invention are not so limited; otherconstructs can be used to describe color, such as the other referencesmentioned herein. In one embodiment, a dominant wavelength of a bluelight source used in a system to produce the first color gamut is in therange 405-410 nanometers and conforms to the Blue-ray Disk specificationand the second color gamut is based on a blue light wavelength in rangeof 450-460 nanometers.

For purposes of discussing and understanding the embodiments of theinvention, it is to be understood that various terms are used by thoseknowledgeable in the art to describe techniques and approaches.Furthermore, in the description, for purposes of explanation, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present invention. It will be evident, however, toone of ordinary skill in the art that the present invention may bepracticed without these specific details. In some instances, well-knownstructures and devices are shown in block diagram form, rather than indetail, in order to avoid obscuring the present invention. Theseembodiments are described in sufficient detail to enable those ofordinary skill in the art to practice the invention, and it is to beunderstood that other embodiments may be utilized and that logical,mechanical, electrical, and other changes may be made without departingfrom the scope of the present invention.

Some portions of the description may be presented in terms of algorithmsand symbolic representations of operations on, for example, data bitswithin a computer memory. These algorithmic descriptions andrepresentations are the means used by those of ordinary skill in thedata processing arts to most effectively convey the substance of theirwork to others of ordinary skill in the art. An algorithm is here, andgenerally, conceived to be a self-consistent sequence of acts leading toa desired result. The acts are those requiring physical manipulations ofphysical quantities. Usually, though not necessarily, these quantitiestake the form of electrical or magnetic signals capable of being stored,transferred, combined, compared, and otherwise manipulated. It hasproven convenient at times, principally for reasons of common usage, torefer to these signals as bits, values, elements, symbols, characters,terms, numbers, or the like.

It should be borne in mind, however, that all of these and similar termsare to be associated with the appropriate physical quantities and aremerely convenient labels applied to these quantities. Unlessspecifically stated otherwise as apparent from the discussion, it isappreciated that throughout the description, discussions utilizing termssuch as “processing” or “computing” or “calculating” or “determining” or“displaying” or the like, can refer to the action and processes of acomputer system, or similar electronic computing device, thatmanipulates and transforms data represented as physical (electronic)quantities within the computer system's registers and memories intoother data similarly represented as physical quantities within thecomputer system memories or registers or other such information storage,transmission, or display devices.

An apparatus for performing the operations herein can implement thepresent invention. This apparatus may be specially constructed for therequired purposes, or it may comprise a general-purpose computer,selectively activated or reconfigured by a computer program stored inthe computer. Such a computer program may be stored in a computerreadable storage medium, such as, but not limited to, any type of diskincluding floppy disks, hard disks, optical disks, compact disk-readonly memories (CD-ROMs), and magnetic-optical disks, read-only memories(ROMs), random access memories (RAMs), electrically programmableread-only memories (EPROM)s, electrically erasable programmableread-only memories (EEPROMs), FLASH memories, magnetic or optical cards,etc., or any type of media suitable for storing electronic instructionseither local to the computer or remote to the computer.

The algorithms and displays presented herein are not inherently relatedto any particular computer or other apparatus. Various general-purposesystems may be used with programs in accordance with the teachingsherein, or it may prove convenient to construct more specializedapparatus to perform the required method. For example, any of themethods according to the present invention can be implemented inhard-wired circuitry, by programming a general-purpose processor, or byany combination of hardware and software. One of ordinary skill in theart will immediately appreciate that the invention can be practiced withcomputer system configurations other than those described, includinghand-held devices, multiprocessor systems, microprocessor-based orprogrammable consumer electronics, digital signal processing (DSP)devices, set top boxes, network PCs, minicomputers, mainframe computers,and the like. The invention can also be practiced in distributedcomputing environments where tasks are performed by remote processingdevices that are linked through a communications network.

The methods herein may be implemented using computer software. Ifwritten in a programming language conforming to a recognized standard,sequences of instructions designed to implement the methods can becompiled for execution on a variety of hardware platforms and forinterface to a variety of operating systems. In addition, the presentinvention is not described with reference to any particular programminglanguage. It will be appreciated that a variety of programming languagesmay be used to implement the teachings of the invention as describedherein. Furthermore, it is common in the art to speak of software, inone form or another (e.g., program, procedure, application, driver, . .. ), as taking an action or causing a result. Such expressions aremerely a shorthand way of saying that execution of the software by acomputer causes the processor of the computer to perform an action orproduce a result.

It is to be understood that various terms and techniques are used bythose knowledgeable in the art to describe communications, protocols,applications, implementations, mechanisms, etc. One such technique isthe description of an implementation of a technique in terms of analgorithm or mathematical expression. That is, while the technique maybe, for example, implemented as executing code on a computer, theexpression of that technique may be more aptly and succinctly conveyedand communicated as a formula, algorithm, or mathematical expression.Thus, one of ordinary skill in the art would recognize a block denotingA+B=C as an additive function whose implementation in hardware and/orsoftware would take two inputs (A and B) and produce a summation output(C). Thus, the use of formula, algorithm, or mathematical expression asdescriptions is to be understood as having a physical embodiment in atleast hardware and/or software (such as a computer system in which thetechniques of the present invention may be practiced as well asimplemented as an embodiment).

A machine-readable medium is understood to include any mechanism forstoring or transmitting information in a form readable by a machine(e.g., a computer). For example, a machine-readable medium includes readonly memory (ROM); random access memory (RAM); magnetic disk storagemedia; optical storage media; flash memory devices; electrical, optical,acoustical or other form of propagated signals (e.g., carrier waves,infrared signals, digital signals, etc.); etc.

As used in this description, “one embodiment” or “an embodiment” orsimilar phrases means that the feature(s) being described are includedin at least one embodiment of the invention. References to “oneembodiment” in this description do not necessarily refer to the sameembodiment; however, neither are such embodiments mutually exclusive.Nor does “one embodiment” imply that there is but a single embodiment ofthe invention. For example, a feature, structure, act, etc. described in“one embodiment” may also be included in other embodiments. Thus, theinvention may include a variety of combinations and/or integrations ofthe embodiments described herein.

While the invention has been described in terms of several embodiments,those of skill in the art will recognize that the invention is notlimited to the embodiments described, but can be practiced withmodification and alteration within the spirit and scope of the appendedclaims. The description is thus to be regarded as illustrative insteadof limiting.

1-78. (canceled)
 79. A method comprising: applying a correction to animage signal, wherein the correction is a function of a modulation levelof a light source used to display an image.
 80. The method of claim 79,wherein the correction is a gamma correction.
 81. The method of claim79, wherein a separate correction is applied to each light source usedto display the image.
 82. The method of claim 79, wherein the lightsource is one of a light emitting diode, an edge emitting light emittingdiode, a laser diode, a diode pumped solid state laser and a laser lightsource.
 83. The method of claim 79, wherein the light source isexternally modulation.
 84. A method comprising: applying a differentgamma correction to each light source within a display, wherein thedisplay is used to display an image.
 85. The method of claim 84, whereineach gamma correction is a function of a modulation level of each lightsource.
 86. The method of claim 84, wherein the light source isexternally modulated.
 87. An apparatus comprising: a display havinglight sources, the light sources are configured to display individualcolors of an image signal, wherein separate gamma corrections areassociated with each light source such that a white balance of thedisplay is maintained across a range of light source drive levels. 88.The apparatus of claim 87, wherein each gamma correction is a functionof a modulation level of the light sources.
 89. A computer readablemedium containing executable computer program instructions, which whenexecuted by a data processing system cause the data to processing systemto perform a method comprising: applying a correction to an imagesignal, wherein the correction is a function of a modulation level of alight source used to display an image.
 90. The computer readable mediumof claim 89, wherein the correction is a gamma correction.
 91. Thecomputer readable medium of claim 89, wherein a separate correction isapplied to each light source used to display the image.
 92. The computerreadable medium of claim 89, wherein the light source is one of a lightemitting diode, an edge emitting light emitting diode, a laser diode, adiode pumped solid state laser and a laser light source.